Question: The lifespans of gorillas in a particular zoo are normally distributed. The average gorilla lives $22.4$ years; the standard deviation is $5.4$ years. Use the empirical rule (68-95-99.7%) to estimate the probability of a gorilla living longer than $6.2$ years.
Solution: $22.4$ $17$ $27.8$ $11.6$ $33.2$ $6.2$ $38.6$ $99.7\%$ $0.15\%$ $0.15\%$ We know the lifespans are normally distributed with an average lifespan of $22.4$ years. We know the standard deviation is $5.4$ years, so one standard deviation below the mean is $17$ years and one standard deviation above the mean is $27.8$ years. Two standard deviations below the mean is $11.6$ years and two standard deviations above the mean is $33.2$ years. Three standard deviations below the mean is $6.2$ years and three standard deviations above the mean is $38.6$ years. We are interested in the probability of a gorilla living longer than $6.2$ years. The empirical rule (or the 68-95-99.7 rule) tells us that $99.7\%$ of the gorillas will have lifespans within 3 standard deviations of the average lifespan. The remaining $0.3\%$ of the gorillas will have lifespans that fall outside the shaded area. Because the normal distribution is symmetrical, half $({0.15\%})$ will live less than $6.2$ years and the other half $({0.15\%})$ will live longer than $38.6$ years. The probability of a particular gorilla living longer than $6.2$ years is ${99.7\%} + {0.15\%}$, or $99.85\%$.